1. Field of Invention
The present invention relates to the technical field of the pyrogenic generation of gases which can be used in particular in systems for protecting the occupants of a motor vehicle by means of bags which are inflated by the combustion gases from a pyrotechnic charge. More specifically, the invention relates to pyrotechnic compositions which generate clean and nontoxic gases at temperatures acceptable for motor vehicle safety. The invention also relates to a process for the continuous manufacture of such compositions.
2. Description of Related Art
For various pyrotechnic requirements and in particular to provide correct inflation of airbags, pyrotechnic gas generators have to provide, in extremely short times, of the order of thirty milliseconds, gases which are clean, that is to say devoid of solid particles capable of constituting hot points which may damage the wall of the bag, and nontoxic, that is to say with low contents of nitrogen oxides, of carbon oxides and of chlorinated products.
Various families of pyrotechnic compositions have been developed for this purpose.
A first family relates to compositions based on alkali metal azide or alkaline earth azide in the presence of an inorganic oxidizing agent, such as potassium nitrate, or of a metal oxide. These compositions, which may if appropriate comprise a binder, exhibit major disadvantages. First, they produce, during their combustion, a good deal of dust, which has to be filtered out using relatively large filtration systems, which increases both the weight and the cost of the generator. Secondly, the azides are highly toxic products which additionally exhibit the possibility of forming azides of lead or of other heavy metals, which are primary explosives. These compositions are therefore difficult to store under satisfactory conditions for several years in a motor vehicle.
A second family relates to compositions based on nitro-cellulose and on nitroglycerine. These compositions, also known under the name of xe2x80x9cdouble-base powdersxe2x80x9d, are highly advantageous as they burn very quickly and without producing dust. However, they nonetheless exhibit the disadvantage of not being completely stable over time, and at high temperature.
A third family relates to xe2x80x9ccompositexe2x80x9d compositions composed essentially of an organic binder and of an oxidizing inorganic filler, such as, in particular, an inorganic perchlorate. These compositions are a priori highly advantageous as they exhibit a good rate of combustion and an excellent stability with regard to ageing.
Compositions have thus been provided, by Patent FR-A-2 137 619 or by its equivalent U.S. Pat. No. 3,723,205, in which the binder is a poly(vinyl chloride) and in which the oxidizing filler is an ammonium perchlorate, in the presence of sodium nitrate as internal chlorine scavenger. Nevertheless, the use of a chlorinated binder in the presence of energy-generating fillers is a problematic operation, in particular with regard to safety and the nontoxicity of the gases generated.
Provision has also been made for composite compositions composed of a silicone binder which can crosslink at ambient temperature, also known under the name of RTV (Room Temperature Vulcanizable), and of potassium perchlorate, the potassium atom acting as internal chlorine scavenger. Such compositions are, for example, disclosed in Patents FR-A-2 190 776 and FR-B-2 213 254 or in their United States equivalents U.S. Pat. No. 3,986,908 and U.S. Pat. No. 3,964,256. However, these compositions exhibit the disadvantage of generating gases which are very rich in oxygen, which gases are not desirable for manufacturers in the motor vehicle industry.
There also exist composite compositions composed of a silicone binder and of a mixture of ammonium perchlorate and of sodium nitrate. Such compositions do not comprise a solvent. They are, for example, disclosed in French Patent FR-A-2 728 562 or in its United States equivalent U.S. Pat. No. 5,610,444. These compositions certainly generate clean, nitrogen-rich and nontoxic gases but exhibit the disadvantage of burning at very high temperatures and of producing a high level of solid waste.
The processes for the manufacture of the existing compositions involve the presence of a solvent for adjusting the viscosity. The use of a solvent has many disadvantages, in particular industrially. The solvent has to be removed from the composition and there is a risk of the pyrotechnic charge becoming porous during this operation.
A person skilled in the art is therefore constantly looking for solvent-free gas-generating pyrotechnic compositions which generate, at temperatures acceptable for the motor vehicle industry, clean and nontoxic gases with very little solid waste. A person skilled in the art is also looking for a process for the continuous manufacture of such compositions, in particular in the form of blocks.
The object of the present invention is specifically to provide such compositions and a process which allows them to be prepared.
The invention thus relates to a gas-generating pyro-technic composition comprising a binder, a nitrogenous organic compound, additives and an oxidizing filler comprising ammonium perchlorate and a chlorine scavenger, the said binder being a hydrocarbonaceous binder with at least two components, one of the components being composed of a gum, characterized in that, when the gum is a polyester gum, it is used in combination with a polyester resin and in that, when the gum is an acrylic gum, it is used in combination with one of its plasticizers.
The term xe2x80x9cgumxe2x80x9d is used to describe a polymer with a molecular mass of greater than 200 000. The acrylic gums used are also known as acrylic rubbers or poly-acrylates. These gums can have reactive endings of chlorine/carboxyl, chlorine, hydroxyl or epoxy type.
The polyester gums used are rubbers with ester units which can have reactive endings of hydroxyl type.
The term xe2x80x9cresinxe2x80x9d is used to describe a hydrocarbonaceous polymer with a molecular mass of between 100 and 10 000.
According to a first preferred embodiment, the binder is composed of the combination of an acrylic gum and of one of its plasticizers. The plasticizer of the acrylic gum is chosen from the group consisting of dioctyl adipate and dioctyl azelate.
A crosslinking agent will generally be used in combination with the binder.
According to a second preferred embodiment, the binder is composed of the combination of a polyester gum and of a polyester resin. According to this preferred embodiment, the composition additionally comprises a crosslinking agent of isocyanate type.
According to a third preferred embodiment, the content by weight of the fillers is greater than or equal to 85% of the total weight of the composition. The term xe2x80x9cfillersxe2x80x9d is understood to mean simultaneously the oxidizing fillers, the nitrogenous organic compounds and the other additives.
The oxidizing filler comprises ammonium perchlorate and a chlorine scavenger. The chlorine scavenger is chosen from the group consisting of sodium nitrate, calcium carbonate, lithium carbonate, potassium nitrate, strontium nitrate, barium nitrate, potassium chlorate, potassium perchlorate and copper oxide.
Sodium nitrate is a preferred chlorine scavenger.
The composition also comprises a nitrogenous organic compound. The nitrogenous organic compound is chosen from the group consisting of nitroguanidine, guanidine nitrate, aminoguanidine nitrate, oxamide, dicyandiamide, guanylurea dinitramide and metal cyanamides. The content by weight of this nitrogenous compound is preferably between 3 and 15% of the total weight of the composition.
According to a fourth preferred embodiment, the composition additionally comprises a ballistic catalyst chosen from the group consisting of titanium oxide, copper oxide, basic copper nitrate, copper chromite and iron oxide. Iron oxide is the preferred ballistic catalyst. The content by weight of the ballistic catalyst is preferably between 0% and 4% of the total weight of the composition.
It makes it possible, inter alia, to improve the rate of combustion.
According to a fifth preferred embodiment, the composition additionally comprises a wetting agent. This wetting agent is chosen from the group consisting of organosilanes, titanates and aziridines. Trialkyl-silanes in which the functional group is a vinyl, epoxy, amine or methacrylic group are the preferred organosilanes.
The content by weight of the wetting agent is preferably between 0.5 and 2% of the total weight of the composition.
This component makes it possible to reduce the residual porosity of the product.
The ballistic catalyst and the wetting agent constitute the preferred additives of the compositions according to the invention.
The invention also relates to a process for the continuous and solvent-free manufacture in a twin-screw mixer-extruder of such compositions, characterized in that:
the twin-screw mixer-extruder comprises a mixing and kneading compartment, a compression compartment and an extrusion head, and in that
the solid and liquid constituents are introduced into the mixing and kneading compartment via two different feed openings, a feed opening for the solids and a feed opening for the liquids, and in that they are, in this compartment, conveyed and kneaded, then, in that,
the homogeneous paste thus formed is degassed in the compression compartment and then extruded, using an extrusion head, in the form of rods and, finally, in that,
the rods thus formed are cut up into charges using a cutting device, and in that these said charges are crosslinked at a temperature of between 100xc2x0 C. and 150xc2x0 C.
According to a preferred alternative form of the invention, the nitrogenous organic compound and the gum are premixed and introduced into the mixing and kneading compartment via the feed opening for the solids.
According to another preferred alternative form of the invention, the pressure in the compression compartment is less than 50xc3x97103 Pa, i.e. 500 mbar.
The temperature in the mixing and kneading compartment is between 15xc2x0 C. and 75xc2x0 C.
The pressure in the extrusion head is between 6xc3x97106 Pa and 15xc3x97106 Pa, i.e. between 60 bar and 150 bar.
The essential novel feature of the invention lies in the fact that the hydrocarbonaceous binder comprises, first, a gum and, secondly, a liquid constituent which is either a resin or a plasticizer. A pasty binder is thus obtained. When the oxidizing filler, the nitrogenous organic compound and the various additives are incorporated in this binder, the composition is sufficiently strong to be extruded in the form of rods. There is therefore no need either for a thickening agent or for a solvent.
The rods are subsequently cut up into charges and the structure of the binder is permanently fixed by crosslinking in an oven at a temperature of between 100xc2x0 C. and 150xc2x0 C.